Computers generally use a combination of a memory based on semiconductor technology (a RAM) with a disk memory based on magnetic recording technology. The RAM features greatest speed for accessing and the magnetic memory features large volume. The problem with the RAM is that it is a volatile memory, i.e., when the computer is turned off either purposely or when a power failure occurs, the contents of the RAM are destroyed, whereas the contents of a magnetic memory are not lost when power is discontinued. Consequently, if it is desired to save data stored in RAM based storage unit when power is shut off, then the operator must intentionally first transfer the data to a disk based storage unit. Sometimes, the operator forgets to make the transfer. Furthermore, transfer of each file from RAM to disk can be tedious if there are a large number of files to be transferred individually.
The procedure of intentional transfer of data from RAM based storage unit to disk based storage unit can not be applied when the power is interrupted inadvertently, e.g., when a power failure occurs. In this case, data stored in RAM based storage unit is lost. In order to resolve the problem of data loss due to inadvertent power failure, a number of devices (circuit packages) have appeared on the market which detect an inadvertent power failure and immediately switch on a battery supply that can sustain the system for several hours until power is restored. The problem with this arrangement is that a power failure can occur when the computer is unattended--such as in the middle of the night or on a weekend--or the cause of the power failure may be such that power is unavailable for an extended period. The result of these situations is that the battery will run down.